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Many cells have an apical and a basal side. For example, neurons and hair cells only have cellular appendages on one side. However, the asymmetry in the cells’ left and right sides is only revealed by molecular analyses of cell membrane proteins. This planar cell polarity is crucial, for example, in the development of ordered organ structures. One of the issues being investigated by researchers led by Prof. Dr. Matias Simons from the University of Freiburg is how the perfectly ordered patterns on the surface of Drosophila wings develop.

The single-layer wall of the lung epithelium shows that cells have a top (apical) side where hairy structures reach into the lung lumen, and a bottom (basal) side that lacks such hairy structures. The majority of epithelia have this kind of apico-basal asymmetry, including the renal channels (tubuli) in which each cell has a sensory cilium that projects into the lumen. These tubuli are rather like sensors that control the composition of urine. Using this model system, Prof. Dr. Matias Simons from the Center for Biological Systems Analysis (ZBSA) at the University of Freiburg discovered another type of cellular asymmetry around 10 years ago. What he discovered was planar cell polarity (PCP) pathways that polarize cells in the tissue plane and equip cells with a defined orientation (left/right axis). Between 2002 and 2005, Simons investigated the causes of polycystic kidney diseases in which the epithelial cells of renal tubuli are unable to divide on their correct axis, which results in the formation of epithelial cysts. It was known that such defects were associated with defective PCP signalling involved in early embryonic development. In cooperation with colleagues in Prof. Dr. Gert Walz’ laboratory at the Freiburg University Medical Centre, Simons found that mutated Inversin genes led to the development of polycystic kidneys. “We also found that the Inversin gene was a determinant of planar cell polarity,” said Simons.

A molecular cassette determines planar cell polarity

The proteins involved in the establishment of planar cell polarity are part of one of the most fundamental signalling networks of embryonic development in the majority of organisms. These proteins also play a key role in adult organisms. For example, the proteins control the directed migration of cells as the neural tube in embryos closes, thereby creating an early form of the central nervous system. While it is known that these proteins are somehow connected with the cytoskeleton, details of these connections are not yet known. For example, they are involved in controlling the synchronous movements of cilia on millions of lung cells that clear the lungs of mucus. The correct formation of a planar left/right axis is also critical for the coordinated orientation of cells and tissues such as skin, the intestines and the brain. “All cells in tissues need to know their correct position,” said Simons going on to add, “defective PCP signalling contributes to many diseases including polycystic kidney disease.” In order to understand the mechanism underlying these signalling processes, Simons went to New York in 2005 to spend his postdoctoral study period in the laboratory of Drosophila specialist Marek Mlodzik. Although his previous work with vertebrates was somewhat closer to clinical aspects, Simons nevertheless shifted his focus to the model system of Drosophila, which is easier to investigate using genetic methods and whose two conserved PCP protein cassettes have been investigated in detail.

Numerous evolutionarily conserved PCP genes are now known, and all of them have been identified in Drosophila mutants. These core PCP genes are involved in establishing molecular asymmetry between and within cells, and they encode Frizzled, Dishevelled, Strabismus and Prickle proteins, which form the classical PCP cassette in and on cellular membranes. This cassette is never found at the apical or basal side of a cell, thereby defining the planar axis that runs from the left to the right side of a cell. In epithelial cells, this axis enables neighbouring cells to join together, thereby making the wall of the intestines, the lungs or renal tubuli impermeable. The cassette is conserved: the gene products of Frizzled and Dishevelled are located on one side of the cell, and the gene products of Strabismus and Prickle on the adjacent side of a neighbouring cell, thereby mediating cell-cell communication. The proteins fit into each other like a key into a lock. During his research stay in New York, Simons worked with researchers from the German Cancer Research Center (DKFZ) in Heidelberg to carry out a genome-wide search for PCP-relevant genes. He also discovered another interesting gene and was able to show that the migration of the Dishevelled protein to the membrane can be interrupted by switching off this particular gene. “We found that this gene coded for a proton transporter,” said Simons referring to the finding that had far-reaching implications.

Wound healing and electrical currents

Subsequent experiments revealed that the recruitment of the Dishevelled protein to its specific destination on the plasma membrane depended on pH gradients (proton concentration). The proton pump discovered by the researchers is a protein complex that is able to remove protons from the cell, thereby altering the pH at specific areas (i.e. areas where Dishevelled normally exerts its function) of the membrane.

Just knowing that a protein that is critical for the development of the correct planar cell axis depends on a proton gradient is particularly significant. However, what is even more important is that the protein gradient at the membrane always leads to changes in the electric membrane potential as well. “We now believe that the PCP signalling networks react to electrical fields and changes occurring in these fields, and vice versa,” said Simons referring to their finding that the pH plays a key role in the establishment of PCP. Together with his team of researchers, Simons is now trying to find the nodes where electrical signals are converted into chemical ones, and vice versa. The researchers’ ultimate goal is to gain an in-depth understanding of which processes are key in the development of organisms and in the maintenance of the ordered structure of adult tissues. Simons hopes that his findings with Drosophila will at some stage be transferable to clinically relevant issues.

It is known that electric currents influence cells and tissues and that they are signals that contribute to regulating cell behaviour during development and repair processes. This is why Simons’ group of researchers at the Center for Biological Systems Analysis (ZBSA), where Simons has been heading up an independent group of researchers since 2009, is also focused on the elucidation of wound healing processes. A wound is simply a crack in the epithelial wall of the skin or another tissue. The ion concentrations of either side of the epithelium differ in closed, i.e. uninjured epithelia. The voltage gradient is crucial for maintaining a transepithelial electric potential. As the epithelial wall is closed, ions cannot migrate and compensate for the difference in potential. If a gap occurs in the cell layer (as a result of injury or other impact), the transepithelial potential breaks down and a planar electric field develops along the epithelial wall at the same time as more distant areas continue displaying intact differences in the electric potential. Are wounds therefore similar to batteries? “We hypothesize that the existence of electric fields precedes the movement or differentiation of cells. The wound closes because cells are able to measure the electric fields. They then migrate along an electrical gradient where they enter and close the gap in the epithelium,” said Simons. “We are now working on achieving an in-depth understanding of the effect of electric fields on cell migration and wound healing.” If the researchers manage to confirm their hypothesis, Simons’ basic research would be of specific clinical use.